This invention relates generally to test instruments for testing communication networks and in particular to a test instrument and method for testing asymmetric digital subscriber lines (ADSL).
ADSL is a new modem technology which converts existing twisted-pair telephone lines into access lines for high speed digital communication and multimedia services such as video on demand (VOD). ADSL operates according to a frequency division multiplex (FDM) scheme in which the frequency spectrum between 0 and 4 kiloHertz (kHz) is allocated for POTS (plain old telephone service) and between 4 kHz and 1.1 MegaHertz (MHz) is allocated for data. In this way, the huge installed base of twisted pair copper wire telephone lines may be used to carry both voice and data traffic. By separating the voice and data paths, the telephone switching system is freed up for conventional voice traffic while the data traffic is moved to digital networks, at data rates 50 times higher than the conventional analog modems over POTS circuits. The POTS circuit is uninterrupted even if the data portion of the ADSL circuit fails.
ADSL is defined according to the American National Standards Institute (ANSI) as ANSI Standard T1.413. An ADSL circuit consists of an ADSL modem on each end of the twisted-pair telephone line. At the telephone company central office, the modem is commonly called the ATU-C (ADSL transmission unit--central) while the modem on the customer premise side is the ATU-R (ADSL transmission unit--remote). ADSL provides data rates up to 8 Megabits per second (Mbps) from the central office to the customer (downstream) while providing upstream data rates from the customer premises up to 640 kbps. This asymmetric relationship between downstream and upstream data rates matches the original intent of ADSL to provide high bandwidth multimedia services downstream with more limited bandwidth requirements upstream from the customer premises.
Twisted-pair telephone lines that will be used for ADSL were designed for low frequency analog voice service, commonly known as POTS. For an installed base of telephone lines, there will be substantial variations in the distance from the customer premises to the central office, the diameter of the copper wires, as well as the number of bridge taps and load coils along each telephone line. ADSL commonly employs Discrete Multitone (DMT) line coding which allows the ATU-R and ATU-C modems to dynamically adapt to the line conditions to obtain the maximum throughput through the ADSL circuit. In addition, ADSL modems may employ forward error correction, trellis encoding, echo cancellation, and other techniques to obtain lower error rate communications, particularly for error sensitive applications such as video transmission. Because of its ability to dynamically adapt to the line conditions of the twisted-pair line, an ADSL circuit must be fully evaluated using traffic generation both upstream and downstream to stress the ADSL circuit to obtain throughput measurements as an overall measure of its performance.
Several types of test instruments exist that may be used in ADSL testing, including line qualification testers and personal computers running test software. Line qualification testers are analog instruments that test the physical condition of the twisted pair line, typically providing such time domain reflectometer (TDR) information as well as attenuation versus frequency and d.c. resistance. However, unless there is a problem with the twisted-pair line that prevents the ADSL circuit from operating properly, the installer of the ADSL circuit cares only about the maximum available throughput through the working ADSL circuit, in units of bits per second (bps), that can be obtained which may be compared against a level of service guaranteed to the customer.
In a conventional digital circuit, the upstream and downstream data rates are the same, allowing a digital loopback test to be performed in which traffic is echoed back from the far end of the digital circuit. However, in an ADSL circuit, the upstream and downstream data rates are different, making the digital loopback test unusable. Furthermore, conventional personal computers having a network interface card (NIC) typically lack the ability to generate traffic at a sufficient number of packets per second to create the level of stress required for this throughput measurement, creating the need for specialized traffic generator circuits capable of generating large amounts of packets per second.
The upstream and downstream data paths in the ADSL circuit, although operating according to the frequency division multiplexing scheme, may interact and interfere with each other to reduce the maximum available throughput. The throughput of the ADSL circuit therefore must be measured with traffic generated in both the upstream and downstream paths simultaneously in order to stress the ADSL circuit, therefore requiring two test instruments that are working on each end of the ADSL circuit in tandem.
Therefore, it would be desirable to provide a test instrument capable of testing an ADSL circuit, operating in tandem with a remote test instrument at the opposite end of the ADSL circuit, to provide a measurement of the throughput of the ADSL circuit.